Currently our efforts are devoted to a detailed analysis of changes in passive stiffness and collagen in hearts hypertrophied by pressure loading and during regression of hypertrophy. Right ventricular hypertrophy will be produced in kittens by banding the pulmonary artery of 12-20 week-old kittens with a clip of sufficient size to elevate right ventricular systolic pressure by 5-15 mm Hg. Passive stiffness will then be assessed at variable periods after banding using isolated right ventricular papillary muscles. Passive stiffness will be measured in slowly stretched non-contracting muscles from stress-strain relationships. The elastic constants are derived from the known exponential relationship of stress and strain. Passive elastic stiffness, the tangent at any point on the stress-strain calculated and compared at various levels of stress. Collagen will be measured as hydroxyprolene in both right and left ventricles. Studies will be performed in groups of animals banded for periods ranging from 2-76 weeks. In addition, the band will be removed from animals banded for comparable periods of time. Studies will then be conducted at variable periods after removal of the band to determine if regression of hypertrophy is associated with changes in stiffness and collagen.